1. Field of the Invention
The present invention relates to an earphone, and more particularly to an earphone suitable for use with a radio receiver in a noisy environment such as an automobile racing circuit, a construction site, or the like.
2. Description of the Prior Art
In automobile racing, conversations between the drivers of racing automobiles and pit members or directors are usually transmitted and received typically through transceivers. The driver hears transmitted conversations with a small-size loudspeaker, a headset, or an earphone which is incorporated in a helmet that the driver wears to protect his head. The noise produced by a racing car while it is running has a very high level of up to 100 through 120 dB. While the helmet has a certain noise insulating capability as it covers the driver's ears, such a high racing noise level is excessive enough to make the helmet ineffective as a noise insulation. Conventional earphones are designed for use with audio systems or in low-noise environments, and cannot be used in noisy environments as the transmitted information that is reproduced by the earphones is masked by the noise.
In view of the aforesaid problems, there have been developed earphones with a noise insulating capability as disclosed in Japanese laid-open utility model publications Nos. 2-21891 and 2-75890, for example.
The earphone disclosed in Japanese laid-open utility model publication No. 2-21891 has an acoustic passage extending from an electroacoustic transducer toward an end to be inserted in an external auditory meatus of the user, the acoustic passage being in the form of an air vibratory system. Since sound produced by the electroacoustic transducer is propagated through the air in the acoustic passage by means of wave motion, external noise may leak through a vibratory plate of the electroacoustic transducer and the acoustic passage into the external auditory meatus.
Japanese laid-open utility model publication No. 2-75890 discloses a headset having a vibration damping material for insulating sound. The headset includes pads for covering the user's ears. When the pads are not properly held against the ears, external noise tends to leak through the headset into the external auditory meatus.
Inasmuch as the conventional earphone or headset is designed to propagate sound waves through air, its noise insulating capability is not sufficient in noisy environments such as automobile racing circuits, construction sites, engine compartments on ships, or the like.